Electrochemical cells, commonly known as “batteries,” are used to power a wide variety of devices used in everyday life. For example, devices such as radios, toys, cameras, flashlights and hearing aids all ordinarily rely on one or more electrochemical cells to operate. Generally, the terms “battery” or “electrochemical cell” are used to describe the connection of one or more electric cells together to convert chemical energy into electrical energy.
Electrochemical cells may be configured as elongate cylindrical cells, such as standard AA-, AAA-, C- and D-sized batteries, which are commonly used in flashlights, portable radios, and toys. Electrochemical cells may also be configured as flat cells, such as prismatic cells and button cells, which are commonly used in watches, hearing aids, and in cordless and cellular telephones.
Conventional primary alkaline electrochemical cells include a negative electrode (anode), a positive electrode (cathode), an electrolyte, a separator, a sealing assembly, a positive current collector, and a negative current collector. These components are typically housed in a battery container, which also functions as the positive current collector, having an open end. The most commonly used cathode of conventional alkaline electrochemical cells comprises manganese dioxide and a conducting carbonaceous material, such as, for example, synthetic graphite, natural graphite, expanded graphite, and mixtures thereof, together with a polymeric binder and other additives. Alkaline electrochemical cells may also comprise other cathode active materials such as NiO, NiOOH, oxides of copper, or mixtures thereof. For example, copper-manganese mixed oxides are excellent candidates for use as cathode active materials in alkaline batteries, either on their own or physically blended with other cathode active materials like manganese dioxide, copper oxide, nickel oxyhydroxide, silver oxides, etc. Synthesis of copper-manganese mixed oxides has previously been disclosed using a variety of processes (see, e.g., U.S. patent application Ser. No. 11/058,665, filed Feb. 15, 2005 and No. 11/354,729, filed Feb. 15, 2006, both of which are incorporated herein by reference).
For utility as active cathode materials in batteries, it is advantageous to use mixed metal oxides with the highest stable oxidation state possible, that provides the mixed oxide cathode having the highest useful discharge voltage and high discharge capacities. To-date, however, how such materials can be synthesized has not been readily apparent, since the starting materials and the process conditions used can significantly impact the resulting oxidation states in the final mixed metal oxide materials containing metal ions at their highest practical oxidation states (for example, copper at the +2 state or higher and manganese the +4 state or higher).
Once prepared, in some alkaline electrochemical cells, the cathode mixture is compressed into one or more annular rings and stacked in the battery container. Alternatively, the mixture may be extruded directly into the battery container.
The anode of conventional alkaline electrochemical cells comprises zinc or zinc alloy particles of various dimensions and shapes along with gelling agents, such as carboxymethylcellulose (CMC), and other additives, such as surfactants. Electrical connection to the anode is achieved by inserting an elongate metal rod, commonly referred to as a negative current collector, pin, or nail, placed in electrical contact with the gelled anode active material. The negative current collector may be made of brass or other suitable metal and extends through a resilient and electrically non-conductive sealing assembly that closes the open end of the battery container, sealing the electrochemical cell components within. The top end of the negative current collector protrudes above the sealing assembly for physical and electrical connection to an electrically conductive negative terminal plate, while the primary length of the negative current collector is inserted into the anode active material within the cell.
In the conventional alkaline electrochemical cell, the cathode is typically formed against the interior surface of the battery container, while the anode is generally centrally disposed in a cavity formed in the center of the cathode. The converse is also possible, where the anode surrounds an inner core of cathode material. To reduce internal resistance and enhance high current discharge, the interior surface of the container is generally coated with a conducting agent, typically comprising carbon. A tubular separator is located between the cathode and the anode. The separator typically extends from the bottom of the battery container to a terminal end extending slightly outward from between the anode and cathode, particularly prior to the cell being closed. The fundamental purpose of the separator is to separate the cathode and anode portions of the alkaline electrochemical cell and prevent an internal short circuit that would compromise the performance or shelf life of the cell. The separator is commonly a multi-layered, permeable, non-woven fibrous material wetted with an alkaline electrolyte. The separator maintains a physical dielectric separation between the anode and cathode, but still allows for the transport of ions and electrolyte between the electrode materials. The separator also acts as a wicking medium for the alkaline electrolyte solution, typically potassium hydroxide or sodium hydroxide, which promotes ionic or electrolytic transport and conductivity. If the anode and cathode come into physical contact with each other in any way, an active chemical reaction occurs, resulting in an internal electrical short circuit or other reduction in the useful electrochemical capacity of the electrochemical cell.
Conventional separators generally comprise non-woven materials that require multiple overlapping layers to prevent unwanted electrical conduction between the cathode and the anode caused by particulate transport or dendrite shorting. Where a single layer of such a separator material is used, openings that are commonly present in the material permit the presence or formation of undesirable conductive paths between the cathode and the anode. Alternatively, the use of multiple or thicker layers of separator material typically increases the volume necessary in the electrochemical cell for accommodating the separator (inactive) component, leaving less room for the active electrochemical materials, and thus potentially reducing the potential life of the cell. The thicker separator materials also tend to increase the amount of ionic resistance between the anode and the cathode, limiting the high rate discharge performance of the electrochemical cell.
Upon closing the cell, the sealing assembly is compressed against the terminal end of the separator, often causing the terminal end of the separator to fold slightly, flare, or even to fold over upon itself, so that the terminal portion of the separator is in contact with the sealing assembly in such a manner as to inhibit the electrode materials from being carried over the terminal end of the separator between the cathode and the anode compartments. Generally, the sealing assembly is formed of a material which is inert to the alkaline electrolyte contained in the cell and the overall environment of the cell itself. The sealing assembly must also be flexible and be able to maintain a proper seal during extended periods of use or storage. Materials such as nylon, polypropylene, ethylene-tetrafluoroethylene copolymer and high density polyethylene are known in the art as suitable sealing assembly materials. While these sealing assemblies help keep the can (electrically connected to the cathode) and the top (electrically connected to the anode) from contacting each other, electrical shorting and loss of battery life may still occur due to the transport of anode or cathode particles over the tope of the separator, resulting from separation of the separator from the sealing assembly during manufacturing, distribution, handling, or use.
As a result of the deficiencies in the thicker separator materials in conventional cells, various thin film and membrane separator materials have been developed. These separator materials function in a similar manner to the thicker separator materials. However, effectively incorporating such materials in cylindrical batteries while maintaining the reliability from shorting is a challenge as compressing these thin film and membrane separator materials against the sealing assembly in the same manner as the conventional thick separators may fail to completely prevent contact between the cathode and the anode during manufacturing, distribution, transportation, handling, or use. Hence it is desirable to develop novel design approaches to produce a robust cell with adequate shorting resistance.